Scanning Probe Microscopes (SPM, including e.g., AFM, SFM, and STM) have long been used, in conjunction with ultra-sharp tips, to move individual atoms or molecules to precise locations. When such site-specific positioning (and force, if necessary) is used to make or break chemical bonds, this is referred to as mechanosynthesis. (Oyabu, Custance et al., “Mechanical vertical manipulation of selected single atoms by soft nanoindentation using near contact atomic force microscopy,” Phys. Rev. Lett., 17, 2003; Morita, Sugimoto et al., “Atom-selective imaging and mechanical atom manipulation using the non-contact atomic force microscope,” J. Electron Microsc., 2, 2004; Oyabu, Custance et al., “Mechanical Vertical Manipulation of Single Atoms on the Ge(111)-c(2×8) Surface by Noncontact Atomic Force Microscopy,” Seventh International Conference on non-contact Atomic Force Microscopy, Seattle, Wash., 2004; Sugimoto, Jelinek et al., “Mechanism for Room-Temperature Single-Atom Lateral Manipulations on Semiconductors using Dynamic Force Microscopy,” Physical Review Letters, 10, 2007; Sugimoto, Pou et al., “Complex Patterning by Vertical Interchange Atom Manipulation Using Atomic Force Microscopy,” Science, 2008).
Conceptually, mechanosynthesis might be broken into several components: A source of atoms or molecules (“feedstock”) used to build (including modifying) workpieces; a place where feedstock is stored while awaiting use (a “feedstock depot”); the product being built (a “workpiece”); a place to store reaction byproducts (a “trash depot”); a structure that directly performs mechanosynthesis reactions (a “tip”); a surface (or “presentation surface”), which can serve several purposes including serving as a feedstock depot and a surface upon which to build the workpiece; and a positional means (e.g., an SPM probe) which controls the relative position of, e.g., tips and workpieces, to facilitate the desired reactions.
Not all systems will have each of these parts as discrete entities, and some will be missing completely. For example, in some mechanosynthesis experiments, one atom was interchanged for another on a surface. In such cases, the presentation surface, feedstock depot, and workpiece were one and the same. In this work, there was no trash depot, since there were no reaction byproducts. Also, note that in some previous examples of such work, only one tip was required because only one or two distinct reactions were being performed (although they may have occurred many times each), and one tip sufficed for all reactions.
However, as the desired reactions become more varied, greater flexibility can be obtained by having a distinct feedstock, feedstock depot, trash depot, presentation surface, and workpiece. Also, multiple tips may be required, each designed to facilitate a particular reaction or set of reactions. Note that a requirement for multiple tips implies some way to bring multiple tips to bear for sequential or parallel operation (e.g., multiple positional means, or some way to swap tips on a single positional means).
As an example of systems that use discrete feedstock, feedstock depots, presentation surfaces, workpieces, and multiple tips, among other possible components, methods for the creation of atomically-precise tips from non-atomically-precise tips (“bootstrapping”) have been described, along with numerous mechanosynthetic reactions which employ a variety of tips, and methods for using multiple reactions to form build sequences for creating complex workpieces (e.g., see patent documents U.S. Pat. No. 9,244,097, US Patent Publication No. 20150355228, US Patent Publication No. 20130184461, US Patent Publication No. 20130178627, US Patent Publication No. 20130178626, U.S. Pat. No. 8,276,211, and U.S. Pat. No. 8,171,568).
Systems capable of more varied build sequences tend to have higher chemical and equipment complexity. For example, chemically, bootstrapping is not a simple process. Neither is the design of new tips, along with the reactions to regenerate tips which are to be used multiple times. Further, feedstock needs to be provided in a chemically-appropriate manner (e.g., feedstock needs to be provided in a manner that will not allow it to react inappropriately with itself, other feedstock, the feedstock depot, or in ways counter to its designated tip binding modes).
In terms of equipment, a larger number of reactions, more types of feedstock, and larger workpieces can all require a larger presentation surface. A larger presentation surface means that the positional means must maintain sub-Angstrom accuracy over longer distances. Additionally, if multiple tips are required, some solution to the problem of using each tip as needed must be provided.
The chemical problems can and have been addressed, as shown by the cited references. And, the equipment problems can all be addressed.
For example, while obtaining the requisite accuracy can be challenging, it is by no means infeasible. Software can be used to enhance the positional accuracy of mechanosynthesis equipment either by correcting for various types of positional errors (Ceria, Ducourtieux et al., “Estimation of the measurement uncertainty of LNE's metrological Atomic Force Microscope using virtual instrument modeling and Monte Carlo Method,” 2015) or through the use of image recognition, allowing the location of a tip to be determined based on the observed surface features. (Lapshin, “Feature-oriented scanning methodology for probe microscopy and nanotechnology,” Nanotechnology, 9, 2004; “Automatic drift elimination in probe microscope images based on techniques of counter-scanning and topography feature recognition,” Measurement Science and Technology, 3, 2007; “Feature-Oriented Scanning Probe Microscopy,” Encyclopedia of Nanoscience and Nanotechnology, 2011; Celotta, Balakirsky et al., “Invited Article: Autonomous assembly of atomically perfect nanostructures using a scanning tunneling microscope,” Rev Sci Instrum, 12, 2014)
Hardware can also be used to refine microscope tip position. Many microscope systems are open-loop, meaning they do not employ metrology to correct tip position. However, closed-loop systems which do employ metrology are available. For example, AttoCube's (attocube systems AG, Muenchen, Germany) attoDRY LAB claims <1 nm sensor resolution with no piezo hysteresis, attained using interferometry. And, multi-tip systems are also available. And, in addition to literature describing the custom fabrication of multi-probe SPMs (Eder, Kotakoski et al., “Probing from both sides: reshaping the graphene landscape via face-to-face dual-probe microscopy,” Nano Letters, 5, 2013), various vendors sell systems that have either more than one probe, or the ability to swap tips on a single probe. For example, the MultiView 4000 (NANONICS IMAGING LTD. HEADQUARTERS, Israel), which can employ up to 4 probes, the “Titanium” (NT-MDT Co., Building 100, Zelenograd, Moscow 124482, Russia), which has a cartridge that can automatically swap between 38 probes, and the LT QuadraProbe™ (RHK Technology, Inc, Troy, Mich. 48083 USA) which includes 4 probes.
However, even though the chemical and equipment challenges inherent in complex mechanosynthesis can be solved, the solutions can increase the cost and complexity of the systems, slow their functioning, and increase the difficulty of designing and manufacturing new tips, reactions, and build sequences. Other solutions, including simply avoiding some of the problems in the first place, would therefore be useful.